I have a 88firebird formula ws6 with a 305tbi/700R4/ and i know its a 10-bolt open diff. I want to beef it up but I need to know if its a 7.5in or 7.65in

 

if its the original rear its probably a 7.625, but that number means nothing for most upgrades.

it only matters if you are swapping the innards of the carrier, because the spacing inside, where the spiders gears/posi unit go, it different by about a quarter inch.

But nobody does that, they just swap out the entire carrier. Unless you are buying a powertrax locker.

 

Around 1986, GM upgraded to 7-5/8". Nothing else is different, just the size of the ring gear was increased. 7-1/2 and 7-5/8 components are the same.

Even though the ring gear is 7-5/8, the diff is still called a 7-1/2".

Yours will have 26 spline axles unless a previous owner upgraded to 28 spline.

 

yea its all original, I'm just looking to swap out the differential for a detroit truetrack a 3.27 gears and some street/strip axles. will i be ok if i got all these parts for 7.65? and will either spline fit? I aslong as they match? i was thinking about 28 spline
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so i can buy either 7.5 or 7.65 and be ok?

 

Yes either carrier size will work, but you will need a 26 Spline as a must.

 

what if i buy a 28 splie carrier and 28 spline axles?

 

That will work... That is what I would do actually.

 

You'll need 28 spline axles specific to a third gen. 90-92 third gens got 28 spline axles but any aftermarket ones for 82-92 will work. There are no other factory 28 spline axles which will be the proper length.

 

alright guys i think i soaked it all in thanx for all the help

 

So, if you have a 7.5 rear you can go ahead and purchase a carrier meant for the 7.625 and use said 28 spline axles? Seeing that summit doesn't offer a 28 spline carrier for the 7.5... Sorry for my noobness, but just started looking into this.

 

The carriers, and absolutely everything else except the gears themselves are exactly the same. For all intents and purposes, the 7.5 and 7.625 are one and the same.

 

Just a heads up for my rearend build i ended up using axles from Yukon, a eaton posi unit, and richmond gear set (3:27) and finished it off with a TA performance cover w girdle. while i had the axels out i upgraded the rear brakes with a package from hawksthirdgenparts.com

 

Actually, if you want to get technical about it, you upgraded to 3.23 ratio. The 3.27 ratio is only in the Borg-Warner 9 bolt rears. 3.23 is the 10 bolt ratio.